GEMINGAS SOFT-X-RAY EMISSION AND THE STRUCTURE OF ITS SURFACE

We present a model to explain the decrease in the amplitude of the pul se profile with increasing energy observed in Geminga's soft X-ray sur face thermal emission. We assume the presence of plates surrounded by a surface with very distinct physical properties: these two regions em it spectra of very distinct shapes which present a crossover, the warm plates emitting a softer spectrum than the colder surrounding surface . The strongly pulsed emission from the plates dominates at low energy while the surrounding's emission dominates at high energy, producing naturally a strong decrease in the pulsed fraction. In our illustrativ e example, the plates are assumed to be magnetized, while the rest of the surface is field free. This plate structure may be seen as a schem atic representation of a continuous but very nonuniform distribution o f the surface magnetic field or as a quasi-realistic structure induced by past tectonic activity on Geminga.